Field of the Invention
The invention relates to a device for recovering waste heat energy, to a utility vehicle, and to a method for recovering waste heat energy.
Description of the Background Art
Devices and methods are known from the prior art, for example, it is known that a Clausius-Rankine circuit can be used to at least partly reclaim the waste heat energy of an internal combustion engine particularly for the purpose of improving fuel efficiency in an internal combustion engine of a motor vehicle, in particular a utility vehicle. A system used for this purpose is frequently also called a WHR (waste heat recovery) system. The key components of a WHR system of this type based on a Clausius-Rankine circuit and having a line system conveying a working medium are: one or more vaporizers in which the working medium can be vaporized; an expansion machine in which the pressurized vaporous working medium is expanded. In this process the working medium performs work, where the exerted output can be used in the motor vehicle through direct mechanical action or indirectly as electricity following conversion by means of a generator. A condenser is additionally provided which returns the vaporous working medium to liquid form downstream following expansion in the expansion machine. In this context, the condenser can either be operated using air as cooling medium or can be integrated into the vehicle cooling circuit and thus use the internal combustion engine coolant as cooling medium. Alternatively, a separate cooling circuit can be used for cooling the condenser. Another key component is a delivery pump for conveying and compressing the working medium within the line system. To ensure the reliable operation of the delivery pump, particularly as protection against cavitation, the working medium must absolutely be supercooled specifically at the delivery pump intake. In addition, sensors, control valves and oftentimes also a compensation tank for storing additional working medium are present. A number of different systems already exist which ensure a required or desired supercooling of the working medium at the delivery pump, specifically in front of a working medium intake opening of the delivery pump.
For instance, a WHR system of this type or the line system thereof can be designed as an open system to realize a Clausius-Rankine circuit. In an open system of this type, the compensation tank is left open to the environment so that the condensation pressure is always equal to ambient pressure. To ensure the required supercooling of the working medium at each operating phase of the motor vehicle in an open system of this type, an inlet temperature, for example of a coolant flowing through the condenser of a cooling system provided for the purpose stated above must be appropriately low, i.e. the inlet temperature must be lower than the condensation temperature minus the desired supercooling. In this situation the working medium exits the condenser with a supercooling solely dependent on coolant temperature. Because, however, the maximum installation space available in the motor vehicle and thus the maximum performance capacity of a cooling system are limited, an open system of this type does not always permit a required supercooling of the working medium, particularly at a working medium intake opening of a delivery pump, to always be reliably ensured at all ambient temperatures. At higher ambient temperatures, there is the danger that the WHR system has to be slowed down or even completely deactivated. However, one advantage of this system is that the shutting down of the motor vehicle and the cooling down of the engine and the powertrain elements thereof does not result in a critical negative pressure in the line system for realizing the Clausius-Rankine circuit. This prevents the risk of contamination of the WHR system through the permeation of air for example at seals, pipe connections, exit shaft of the expansion machine, etc. to the greatest extent possible.
If, on the other hand, a WHR system of this type or the line system thereof for realizing the Clausius-Rankine circuit is designed as a closed system, it can reliably be hermetically sealed against the environment. This allows different condensation pressure levels to be set in the condenser, it being generally recommended to set an operating point on the appropriate vapor pressure curve. This operating point is based on the particular combination of condensation pressure and condensation temperature. No supercooling is initially present at the working medium outlet of the condenser. To achieve a desired supercooling of the working medium in the closed system, particularly at the working medium outlet of the condenser, two options can be considered in principle. On the one hand, the desired or intended supercooling can be achieved by means of a supercooling segment connected downstream from the condenser. However, a supercooling likewise connected downstream from the condenser is also required which in general must be operated with a colder cooling medium than the condenser connected upstream to always reliably ensure supercooling. On the other hand, the desired or intended supercooling can be achieved with a generally available high filling quantity of working medium in the line system, which increases average density and thereby ultimately the pressure within the line system, which allows the achievable supercooling to be increased. A general problem in a hermetically closed system of this type is posed, however, by the relatively high negative pressures, which occur as soon as the Clausius-Rankine circuit is not running. These relatively high negative pressures often reach several 100 mbar of relative negative pressure. In this case it is highly doubtful that the system will remain permanently sealed against the environment. Air penetrating the line system due to leakiness poses a problem, since this would cause a change in system pressure such that the achievable supercooling and the penetrating air could compromise the performance capacity of the WHR system.
For example, the patent document DE 10 2009 050 068 A1 describes an internal combustion engine having a cooling circuit and a Clausius-Rankine circuit for waste heat recovery in which the Clausius-Rankine circuit is connected to the cooling circuit via a heat exchanger device in such a way that the waste heat of the cooling circuit can heat and thus vaporize a working medium circulating in the Clausius-Rankine circuit. The Clausius-Rankine circuit comprises a condenser, a pump, an expansion unit, as well as a compensation tank that is always open to the Clausius-Rankine circuit for compensating fluctuations in the volume and/or pressure of the working medium circulating within the Clausius-Rankine circuit. The compensation tank is characterized by a primary chamber in which a portion of the volume of the circulating working medium is present. The compensation tank additionally features a secondary chamber separated from the primary chamber by a membrane in which compressed air can be supplied. Fluctuations in volume and/or pressure of the working medium circulating in the Clausius-Rankine circuit can be compensated through the pressure conditions prevailing in the secondary chamber. For this purpose the secondary chamber is functionally connected to a pressure control device which actively regulates the working medium pressure prevailing in the compensation tank, thereby allowing in general the efficiency of the Clausius-Rankine circuit and thus the efficiency of the expansion unit in particular to be influenced.
Another patent document DE 2011 117 054 A1 furthermore discloses a device for recovering energy from a waste heat flow of an internal combustion engine in a motor vehicle with a working medium circuit. A Clausius-Rankine circuit process takes place within the working medium circuit, wherein integrated in the working medium circuit is a working medium reservoir characterized by a fill level sensor which allows working medium fill level in the working medium circuit to be ascertained and compared to a prescribed specified fill level. If a critical deviation is detected, an optic, acoustic and/or haptic warning is emitted to the vehicle driver. The working medium reservoir can either be connected to the line system of the working medium circuit via an always open and thus passable dead leg or directly constitute a physical segment of the working medium circuit through which the working medium directly flows.